The present invention relates to optical disk data storage devices as used in the computer industry and video recording industry, and in particular to a focus servo apparatus for use in focusing the laser beam of such recording devices. Even more particularly, this invention relates to apparatus and methods of acquiring and maintaining focus of the laser read/write beam using an astigmatic focus system.
An optical disk used to record information in optical disk data storage devices is typically in the form of a circular disk plate of a transparent material with a thin layer into which a plurality of reflectivity changes are formed on and along a plurality of circular tracks arranged concentrically. The information is defined by the length of the reflectivity changes and the distance between the adjacent reflectivity changes. Information is written onto the disk by creating the reflectivity changes and the information is read from the disk by reflecting a beam off the thin layer and detecting the presence or absence of reflectivity changes along the circular track. A readout of the information recorded on the disk is usually performed by a system directing a light to the surface of the disk, detecting light reflected from the thin layer on the surface and modulated by the reflectivity changes, and demodulating the detected light. In a readout system of this type, a servo mechanism is necessary to control a lens system for focusing the incident light precisely on the reflection surface.
An example of the conventional focus servo mechanism which is used in read out systems is shown in FIG. 1. A light beam emitted from a light source, such as a helium-neon laser, is passed through a beam splitter, reflected off a mirror and condensed by a condenser lens to a point on the recording surface of the disk. The disk is rotated by a motor to create concentric tracks on the surface of the disk, and separate tracks are created by moving the mirror and condenser lens toward and away from the disk hub. The reflected light containing the information recorded on the disk passes through the optical elements in a reverse direction. It passes through the condenser lens, reflects off the mirror, and separates at the beam splitter where a portion travels to the photoelectric conversion element to create an electrical signal.
It is impossible, however, to fabricate the disk with complete flatness and, even if the disk is completely flat, it may impossible to mount it on a shaft of the motor without some tilting. Therefore, when the disk is rotated under such a tilted mounting condition, the disk may be vertically fluctuated, that is, the disk may move toward and away from the condenser lens causing the light to alter in focus. To accurately read out information from the disk, the condenser lens must be moved to initially acquire focus of the light on the surface of the disk, and then the lens must be controlled to maintain this focus as the disk position fluctuates vertically.
In order to acquire and maintain focus in a system of this type, an astigmatic focus system is often used. As shown in FIG. 1, a cylindrical lens is provided to receive a portion of the light from the beam splitter. The light passed through the cylindrical lens is received by a quad detector which comprises four segments, A, B, C, and D as shown in FIG. 2. The segments are arranged such that a straight line connecting the centers of segments A and B and a straight line connecting the centers of segments C and D are perpendicular to each other and one of the straight lines is oriented in the same direction as the longitudinal axis of the cylindrical lens. Because a light beam, after passing through a cylindrical lens, has one focus point for light in a plane including the horizontal axis of the lens and a different focus point for light in a plane including the vertical axis of the lens, contours of light projected onto the light receiving segments A through D of the light receiving element are different, so that the positional relation between the recording surface and the condenser lens can be determined by using the difference in the outputs of the quad detector.
That is, the light receiving surface of the quad detector is positioned so that when the focus point of the condenser lens is at a position in the plane of the recording surface of the disk, the contour of the reflected light after being passed through the cylindrical lens becomes substantially a circle, as shown in FIG. 3A. In case the incident light is focused behind the recording surface as shown by the dotted lines in FIG. 4, that is, the distance between the recording surface and the condenser lens is too small, the contour of the light striking the quad detector is an ellipse as shown in FIG. 4A, with the majority of the light falling on segments C and D. On the other hand, in case the incident light is focused in front of the recording surface, as shown by the dotted lines in FIG. 5, that is, the distance between the condenser lens and surface is too long, the light striking the quad detector creates an ellipse with the majority of the light falling on segments A and B as shown in FIG. 5A. Accordingly, if the light is properly focused on the disk surface, the output of all four segments of the quad detector will be equal. However, if the light is focused behind the disk surface the output of segments C and D will be larger than the output of segments A and B, and if the light is focused in front of the recording surface, the output of segments A and B will be larger than the outputs of segments C and D. This information can be used to control the movement of the condenser lens and keep the light properly in focus on the surface of the disk.
While this concept works to maintain focus of the light on the disk, acquiring focus initially requires a somewhat different method. The error signal output of a typical prior art quad detector signal processing circuit as shown in FIG. 6, produces the waveform shown in FIG. 7A. This signal is used to acquire as well as to maintain focus. The waveform shown in FIG. 7B is the summation of the signal from all four elements of the quad detector. Proper focus occurs at the point where the waveform in FIG. 7B peaks, which is also the crossover point of the waveform in FIG. 7A. Because the peak of the waveform in FIG. 7B is a very broad peak, it alone is not sufficient for acquiring focus. Proper focus is acquired when the signal illustrated by the waveform of FIG. 7B is above the threshold shown in FIG. 7B and also the signal shown in FIG. 7A is between the two thresholds shown. Because the waveform in FIG. 7A must be between the thresholds, when the condenser lens is moving in one direction, the positive threshold must be checked whereas if the lens is moving in the other direction, the negative threshold must be checked. Therefore, acquisition of focus in typical prior art devices requires the measuring of three separate thresholds. In addition, since the waveform of FIG. 7B is the sum of the voltages from all four segments, the amount of reflectivity from the disk affects where this threshold has to be placed. That is, a disk that reflects more light requires a higher threshold, and a disk that reflects less light requires a lower threshold. Because of this, the threshold for the signal is FIG. 7B has to be a variable threshold calculated for each individual disk.
The problems of conventional focus servo apparatus as described and shown in FIGS. 1-7 are that focus acquisition requires the measuring and detection of three thresholds having both positive and negative values. In addition, one of these threshold values has to be calculated for each disk since it is sensitive to reflectivity changes from one disk to another. Therefore, there is need in the art for an improved focus acquisition and maintenance system that requires only one threshod and is insensitive to reflectivity changes in the disk media.